Purification and molecular characterization of rat intestinal brush border membrane dipeptidyl aminopeptidase IV

Tri- and dipeptides identification in whey protein and porcine liver protein hydrolysates by fast LC-MS/MS neutral loss screening and de novo sequencing

We describe a fast (5 min) liquid chromatography tandem mass spectrometry method (LC-MS/MS) based on a 46 Da neutral loss of formic acid (H₂ O and CO) to identify tri- and dipeptides (DIPEP) in whey protein and porcine liver protein hydrolysates and confirmed by further de novo sequencing. Sample solutions were acidified to favor [dipep + H]⁺ ions, and a m/z range of 50-300 was used to improve sensitivity. All dipeptide candidates were selected based on all possibilities of the 20 amino acid combinations, and their collision-induced dissociation fragments were screened via de novo sequencing. To determine their biological activities, sequenced dipeptides were compared with the Biopep database and other data from literature. Altogether, 18 dipeptides and 7 tripeptides were identified from the whey protein hydrolysate; they seemed to be broadly active, and peptides were identified as active dipeptidyl peptidase IV inhibitors and active angiotensin-converting enzyme (ACE), according to available information. Porcine liver hydrolysate showed 14 dipeptides which exhibit similar biological activities to whey protein hydrolysate.

Dipeptidyl peptidase 4 - An important digestive peptidase in Tenebrio molitor larvae

Dipeptidyl peptidase 4 (DPP 4) is a proline specific serine peptidase that plays an important role in different regulatory processes in mammals. In this report, we isolated and characterized a unique secreted digestive DPP 4 from the anterior midgut of a stored product pest, Tenebrio molitor larvae (TmDPP 4), with a biological function different than that of the well-studied mammalian DPP 4. The sequence of the purified enzyme was confirmed by mass-spectrometry, and was identical to the translated RNA sequence found in a gut EST database. The purified peptidase was characterized according to its localization in the midgut, and substrate specificity and inhibitor sensitivity were compared with those of human recombinant DPP 4 (rhDPP 4). The T. molitor enzyme was localized mainly in the anterior midgut of the larvae, and 81% of the activity was found in the fraction of soluble gut contents, while human DPP 4 is a membrane enzyme. TmDPP 4 was stable in the pH range 5.0-9.0, with an optimum activity at pH 7.9, similar to human DPP 4. Only specific inhibitors of serine peptidases, diisopropyl fluorophosphate and phenylmethylsulfonyl fluoride, suppressed TmDPP 4 activity, and the specific dipeptidyl peptidase inhibitor vildagliptin was most potent. The highest rate of TmDPP 4 hydrolysis was found for the synthetic substrate Arg-Pro-pNA, while Ala-Pro-pNA was a better substrate for rhDPP 4. Related to its function in the insect midgut, TmDPP 4 efficiently hydrolyzed the wheat storage proteins gliadins, which are major dietary proteins of T. molitor.

Gastrointestinal Function

This chapter describes the normal biochemical processes of intestinal secretion, digestion, and absorption. The digestive system is composed of the gastrointestinal (GI) tract, or the alimentary canal, salivary glands, the liver, and the exocrine pancreas. The principal functions of the gastrointestinal tract are to digest and absorb ingested nutrients, and to excrete waste products of digestion. Most nutrients are ingested in a form that is either too complex for absorption or insoluble, and therefore, indigestible or incapable of being digested. Within the GI tract, much of these substances are solubilized and further degraded enzymatically to simple molecules, sufficiently small in size, and in a form that permits absorption across the mucosal epithelium. This chapter explains in detail the mechanisms of salivary secretions, compositions of saliva, and the functions of saliva. The chapter also elaborates properties of bile as well as the synthesis of bile acids. The chapter explores the pathogenesis of the important gastrointestinal diseases of domestic animals, and the biochemical basis for their diagnosis and treatment. The chapter concludes with a discussion on disturbances of gastrointestinal function such as vomition, acute diarrheas, malabsorption, bacterial overgrowth, and ulcerative colitis.

Dipeptidyl peptidase IV in C6 rat glioma cell line differentiation

The C6 rat glioma cell line is broadly used as a model in studies of glial cell differentiation. In the present study we demonstrated a significantly higher total cellular, but especially membrane-associated, activity of dipeptidyl peptidase IV in differentiated C6 cells in comparison with their proliferating counterparts. The majority, but not all, of enzyme isoelectric focusing isoforms from differentiated C6 cells displayed a substantially higher activity compared to the proliferating cells, with G-P-NHMec as the substrate. Non-denaturing polyacrylamide gradient gel electrophoresis showed the presence of one major peak of activity, dipeptidyl peptidase IV (Mr of about 220000), in both proliferating and differentiated C6 cells. The results indicate that dipeptidyl peptidase IV regulation is associated with C6 rat glioma cell differentiation.

Proline specific peptidases

Proline is unique among the 20 amino acids due to its cyclic structure. This specific conformation imposes many restrictions on the structural aspects of peptides and proteins and confers particular biological properties upon a wide range of physiologically important biomolecules. In order to adequately deal with such peptides, nature has developed a group of enzymes that recognise this residue specifically. These peptidases cover practically all situations where a proline residue might occur in a potential substrate. In this paper we endeavour to discuss these enzymes, particularly those responsible for peptide or protein hydrolysis at proline sites. We have detailed their discovery, biochemical attributes and substrate specificities and have provided information as to the methodology used to detect and manipulate their activities. We have also described the roles, or potential roles that these enzymes may play physiologically and the consequences of their dysfunction in varied disease states.

Human dipeptidyl peptidase IV gene promoter: tissue-specific regulation from a TATA-less GC-rich sequence characteristic of a housekeeping gene promoter

The dipeptidyl peptidase IV gene encodes a plasma-membrane exopeptidase that is highly expressed in small intestine, lung and kidney. In order to better understand the mechanisms responsible for this tissue-specific expression we cloned, sequenced and functionally characterized the 5'-flanking region of the human dipeptidyl peptidase IV gene. The first 500 bases of the 5'-flanking sequence constituted an unmethylated CpG island, contained several Sp1-binding sites and lacked a consensus TATA box, all characteristics of gene promoters lacking tissue-specific expression. RNase-protection analysis using both small intestinal and Caco2 cell RNA indicated that the dipeptidyl peptidase IV transcript was initiated from no fewer than six major and 12 minor start sites. The 5'-flanking sequence also exhibited functional promoter activity in transient transfection experiments. Here, various lengths of the sequence were cloned upstream of a luciferase gene and introduced into cultured cells using lipofectin. A region located between bases -150 and -109 relative to the start of translation was found to be important for high-level promoter activity in both Caco2 and HepG2 cells. Moreover, Caco2 cells and HepG2 cells, which express high levels of dipeptidyl peptidase IV activity, exhibited much higher normalized luciferase activity after transfection than did 3T3, Jurkat or COS-7 cells, which have low enzyme levels. Sodium butyrate was found to increase both enzyme activity and normalized luciferase in HepG2 cells. Thus the dipeptidyl peptidase IV promoter possesses the ability to initiate transcription in a tissue-specific fashion in spite of having the sequence characteristics of a housekeeping gene promoter.

Characterization of microvillar membrane proteins of dog small intestine by two-dimensional electrophoresis

A method for analysing microgram amounts of microvillar membranes by two-dimensional electrophoresis (protein mapping) is described, and has been used to characterize the microvillar proteins of the small intestine of German shepherd, corgi, and beagle dogs. Detergent-solubilized microvillar membranes were radiolabelled with 14C and separated by isoelectric focussing followed by SDS-PAGE. Proteins were detected fluorographically and glycoproteins by lectin-affinity staining. The microvillar hydrolases alkaline phosphatase and dipeptidyl aminopeptidase IV were identified by active-site labelling and aminopeptidase N by immunoprecipitation. Changes following pancreatic duct diversion were consistent with accumulation of pro-sucrase-isomaltase and diminished expression of the sucrase and isomaltase subunits. Cytoskeletal proteins were concentrated in the core fraction remaining after extraction of microvillar membranes with Triton X-100. There were no consistent differences between dogs of different breed, and the canine protein maps were similar to the human.

Oligosaccharide reprocessing and recycling of a cell surface glycoprotein in cultured rat hepatocytes

The metabolism of the cell surface glycoprotein dipeptidyl peptidase IV(DPPIV) was studied in cultured rat hepatocytes. In pulse-chase labelling experiments using L-[35S]methionine a 100-kDa high-mannose precursor polypeptide is converted into the mature complex-type 110-kDa glycoprotein. Digestion with exo- and endoglycosidases and metabolic labelling with radioactive sugars demonstrate that the 110-kDa form contains about 6 complex-type oligosaccharides which are fucosylated and sialylated. About 25 min after the beginning of the pulse-labelled glycoprotein appears in the sinusoidal membrane. Physiologically only the 110-kDa form is found in the cell surface. If cell surface DPP IV was desialylated by sialidase at 4 degrees C, it is resialylated during incubation at 37 degrees C. This oligosaccharide reprocessing indicates that the surface glycoprotein has been recycled to the cell compartment containing terminal glycosyltransferases (presumably the trans Golgi system). Two different methods demonstrate internalization of cell surface DPP IV: 1) The complex cell surface DPPIV -anti-DPP IV-antibody -L-[35S]methionine-labelled secondary goat-anti-mouse-antibody formed at 4 degrees C becomes less accessible to trypsin during incubation at 37 degrees C. 2) Part of the complex plasma membrane DPP IV-anti-DPP IV-antibody formed in the cold cannot be recognized by the radioactive secondary antibody after rewarming. Internalization is not blocked by inhibition of protein synthesis with cycloheximide. During internalization of plasma membrane DPP IV its concentration in the membrane remains constant.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of proline-specific carboxypeptidase localized to brush border membrane of rat small intestine and its possible role in protein digestion

A proline-specific carboxypeptidase (carboxypeptidase P, EC 3.4.12.-) was identified and partially characterized in the brush border membrane fraction of rat intestinal enterocytes and shown to be distinct from pancreatic proteases. The carboxypeptidase activity of isolated brush border membranes, with Z-Gly-Pro-Leu as substrate, was 43 nmol/min/mg protein representing a 16-fold purification when compared with mucosal cell homogenates. Activity was maximal in the middle region of the small intestine, and villus cells had twice the activity of crypt cells. Carboxypeptidase activity was maximal at pH 7.0, was stimulated by divalent cations, and was inhibited by metal chelating agents, suggesting that it is a metalloenzyme. The enzyme had the highest activity with synthetic peptides containing proline penultimate to the carboxy terminus. In vivo patterns of hydrolysis and absorption of amino acids from Z-Pro-Trp were examined using an intestinal perfusion technique. These studies indicate that brush border membrane carboxypeptidase may play an important role in the digestion of proline-containing peptides and proteins.

In vivo effect of tunicamycin on the expression of rat small intestinal brush border membrane glycoproteins and glycoenzymes

Tunicamycin, a known inhibitor of the lipid-dependent glycosylation of proteins, was used in vivo to study the biosynthesis of rat intestinal brush border membrane aminopeptidase N and dipeptidyl aminopeptidase IV. The incorporation of [3H]glucosamine into newly synthesized total protein of mucosal cell homogenates was inhibited by 60%, whereas incorporation of [3H]leucine was decreased only 21% by tunicamycin. This effect was much more pronounced in the brush border membrane fraction isolated from intestinal mucosal cells where incorporation of radiolabled leucine and glucosamine was reduced to 50 and 82% of control values respectively. An examination of the brush border membrane protein profile by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that there was a marked selective decrease in the amount of glycoproteins of molecular weights greater than 130 kD. In addition, there were decreased levels of assayable aminopeptidase N, dipeptidyl aminopeptidase IV and disaccharidase activity in intestinal mucosal cell homogenates and brush border membranes of tunicamycin-treated rats. Though tunicamycin decreased incorporation of newly synthesized aminopeptidase N and dipeptidyl aminopeptidase IV protein into brush border membranes by 70-75%, the newly synthesized enzyme that was incorporated was indistinguishable from that of controls. Further, non-glycoslyated forms of both enzymes were not detected in any other subcellular fractions. These results show that tunicamycin, an inhibitor of glycosylation, significantly affected the expression of brush border membrane glycoproteins, suggesting that both polypeptide synthesis and degradation of these proteins may be altered in the presence of this drug.

Digestion and assimilation of proline-containing peptides by rat intestinal brush border membrane carboxypeptidases. Role of the combined action of angiotensin-converting enzyme and carboxypeptidase P

Two intestinal brush border membrane carboxypeptidases were found to participate in the sequential digestion of proline-containing peptides representing a novel mechanism of hydrolysis from the COOH terminus. NH2-blocked prolyl tripeptides were rapidly hydrolyzed by either brush border membrane angiotensin converting enzyme (ACE, dipeptidyl carboxypeptidase, E.C. 3.4.15.1) or carboxypeptidase P (E.C.3.4.12-) depending on the position of the proline residue. Furthermore, these two enzymes were found to participate in a concerted manner to sequentially degrade larger proline-containing pentapeptides from the COOH terminus. A brush border membrane associated neutral endopeptidase also participated in the hydrolysis of the prolyl pentapeptides. During in vivo intestinal perfusion, the NH2-blocked prolyl peptides were degraded and their constituent amino acids efficiently absorbed by the intestine. Furthermore, hydrolysis and absorption of these peptides could be dramatically suppressed by low concentrations of captopril, a specific inhibitor of ACE. These studies show that prolyl peptides are efficiently and sequentially hydrolyzed from the COOH terminus by the combined action of ACE and carboxypeptidase P, and that these enzymes may play an important role in the digestion and assimilation of proline-containing peptides.

Maturational changes in terminal glycosylation of small intestinal microvillar proteins in the rat

Studies were performed to identify rat intestinal microvillar proteins which undergo changes in terminal glycosylation during postnatal development. Pulse-labeling with [3H]fucose or N-[3H]acetylgalactosamine showed significantly higher incorporation into purified microvillar membranes of weanling than suckling rats. In contrast, the incorporation of [3H]sialic acid after pulse-labeling with N-[3H]acetylmanosamine was higher in suckling rats. SDS-polyacrylamide gel electrophoresis revealed these developmental differences in radioactive sugar incorporation to involve mainly proteins above Mr 90,000. 125I-labeled peanut lectin autoradiography revealed an Mr greater than 330,000 binding protein in suckling rats. Neuraminidase treatment of the membranes revealed the presence of sialyl-substituted sites in this protein in suckling, weaning and weanling animals, but the unmasking of sites decreased with advancing maturation. 125I-labeled Ulex europeus I autoradiography showed marked increases in binding of this lectin to Mr 66,000, 92,000, 130,000, 150,000 and greater than 330,000 proteins from weaning to weanling periods. Similar age-related increases in soybean lectin binding to Mr 130,000-150,000, and greater than 330,000 proteins were demonstrated by affinity chromatography. The Mr values of the major lectin-binding proteins were close to those reported for several hydrolases (trehalase, alkaline phosphatase, sucrase-isomaltase and glucoamylase). Comparison of the Coomassie blue-stained electrophoretograms from each age-group against the corresponding autoradiograms of lection-binding proteins led us to conclude that, while the content of these proteins in the membrane achieve their mature levels at or before weaning, their terminal glycosylation (desialylation, fucosylation, N-acetylgalactosamination) is not fully established until later development.

Identification and characterization of brush-border membrane-bound neutral metalloendopeptidases from rat small intestine

Neutral metalloendopeptidase enzymes were identified and partially characterized in the brush-border membranes of rat small intestinal mucosal cells using insulin B chain and glutaryl-trialanine-4-methoxy-beta-naphthylamide as substrates. Three different molecular species of endopeptidase were identified by disc gel electrophoresis. These enzymes were shown to be distinct from pancreatic endopeptidases on the basis of the following: enrichment in the brush-border membrane fraction, site of hydrolysis of peptide substrates, sensitivity to specific proteinase inhibitors, and the presence of brush-border membrane-associated endopeptidase activity in mucosal cells of Thirty-Vella loops. Hydrolysis of the substrates was shown to be a two-step process involving initial cleavage by endopeptidase with secondary hydrolysis of the peptide products by brush-border membrane aminopeptidase N. Hydrolysis of both substrates was maximum at a neutral pH and was strongly inhibited by metal chelating agents, phosphoramidone, and amastatin. Intestinal perfusion studies using glutaryl-trialanine-4-methoxy-beta-naphthylamide suggest that these enzymes play a physiologic role in protein digestion. It was concluded that neutral endopeptidases are integral components of the intestinal brush-border membrane and work in concert with aminopeptidase N to hydrolyze dietary protein. This process may be of nutritional importance in normal subjects and those with diminished exocrine pancreatic function.

Comparison of brush border membrane glycoproteins and glycoenzymes in the proximal and distal rat small intestine

Brush border membranes isolated from the proximal and distal portions of the rat small intestine were examined to see whether qualitative differences exist in their glycoprotein constituents. After SDS-polyacrylamide gel electrophoresis distinct differences were observed, indicating that the protein and glycoprotein profiles of the distal intestine are less complex. A competitive radioassay of lectin receptors revealed that there are significantly more wheat germ agglutinin and succinylated wheat germ agglutinin receptors present on brush border membranes from proximal intestine as compared to distal intestine. However, binding of Ricinus communis agglutinin I to brush border membranes of distal intestine was 2-times higher than that of proximal intestine. These segmental differences were also reflected in the binding patterns of individual brush border membrane hydrolases to wheat germ agglutinin and R. communis agglutinin I. Carbohydrate analysis demonstrated that the overall sugar content of brush border membranes is higher in distal intestine, with more galactose and sialic acid residues. No difference was found in the content of N-acetylglucosamine between the two segments. When brush border membranes from both segments were used as acceptors for galactosyltransferase, those from proximal intestine were better acceptors. Neuraminidase treatment significantly enhanced galactose oxidase/sodium borotritide labeling of brush border membranes from distal intestine and altered the electrophoretic mobility of dipeptidyl aminopeptidase IV and aminopeptidase N. No significant changes in labeling or enzyme electrophoretic mobility were noted in brush border membranes from proximal intestine after neuraminidase treatment. These studies indicate that the glycoproteins from brush border membranes of proximal and distal intestine are qualitatively different and that the glycoproteins from distal intestine may have more completed oligosaccharide side chains.